The application claims the priorities of Japanese Patent Applications No. 8-186586 filed on Jun. 26, 1996 and No. 9-54016 filed on Feb. 21, 1997.
As the positive electrode for certain secondary batteries such as a nickel-hydrogen storage battery or a nickel-cadmium storage battery, a sintered nickel electrode fabricated by sintering a nickel powder onto a perforated steel substrate or the like and impregnating the resulting plaque with an active material (nickel hydroxide) is well known.
To insure an increased impregnation with the active material for a sintered nickel electrode, it is necessary to employ a sintered substrate or plaque of increased porosity: However, since the inter-particle bond of sintered nickel is weak, increasing the porosity of the substrate increases the tendency for the nickel particles to be dislodged from the plaque. For practical purposes, therefore, the porosity of the sintered substrate cannot be increased beyond 80%, with the result that the sintered nickel electrode has the drawback that the impregnation amount of active material is limited. In addition, since the pore size of the sintered nickel is generally as small as 10 .mu.m or less, impregnation of the plaque with the active material requires the time-consuming dip method which involves several immersion cycles.
For the above reasons, a non-sintered nickel electrode has been proposed of late. The non-sintered nickel electrode is fabricated by impregnating a high-porosity substrate (such as a foamed metal plated with an alkali-resistant metal) with a paste prepared by kneading an active material (nickel hydroxide) and a binder (such as an aqueous solution of methylcellulose) together. Since a high-porosity substrate (with a porosity of 95% or greater) can be used for the non-sintered nickel electrode, not only the impregnation amount of active material can be increased but the impregnation procedure is facilitated.
However, when-a high-porosity substrate is used for increasing the impregnation amount of active material in a non-sintered nickel electrode, the capacity of the substrate as a current collector deteriorates so that the utilization of active material, i.e. utilization efficiency, is decreased. Moreover, the non-sintered nickel electrode has the draw-back that the utilization efficiency at high temperatures is low. Thus, because of its low oxygen over-potential, the charging electric energy in charging is consumed not only in the oxidation reaction from nickel hydroxide to nickel oxyhydroxide but also in the oxygen generating reaction associated with decomposition of water (water in alkaline electrolyte).
Therefore, in order to increase the utilization of active material, i.e. utilization efficiency, in a non-sintered nickel electrode, it has been proposed to use a powdery active material comprising composite particles each consisting of a nickel hydroxide core and a cobalt hydroxide [.beta.-Co(OH).sub.2 or .alpha.-Co(OH).sub.2 ] shell or a powder comprising composite particles each consisting of a nickel hydroxide core and a cobalt oxyhydroxide shell (JP Kokai S62-234867 and JP Kokai H3-78965). Moreover, for assuring a high utilization efficiency over a broad temperature range, it has been proposed to add cobalt metal, cobalt hydroxide, and a yttrium compound to nickel hydroxide powder (JP Kokai H5-28992).
However, the investigations made by the inventors of the present invention revealed that those prior art methods are hardly capable of providing a non-sintered nickel electrode expressing a high utilization efficiency over a large number of charge-discharge cycles.
Furthermore, for enhancing the oxygen overpotential of a non-sintered nickel electrode, a suggestion has been made to add to the active material nickel oxide a specified element (at least one element selected from the group consisting of Ca, Sr, Ba, Cu, Ag, and Y) in the form of a powder of its compound (e.g. Ca(OH).sub.2, CaO, CaS, CaF.sub.2, Y.sub.2 (CO.sub.3).sub.3, Y.sub.2 O.sub.3, etc.) having a mean particle diameter not greater than one-half of the mean particle diameter of said nickel oxide (JP Kokai H8-329937). The object of this technology is to increase the oxygen overpotential of the nickel electrode by adding a compound of such specified element to nickel oxide to thereby suppress the evolution of oxygen in charging.
However, the experimental review by the inventors of the present invention revealed that the above non-sintered nickel electrode also has the following aspects to be improved. Thus, said compound of specified element (e.g. Ca etc.) increases the oxygen overpotential in fact but is little effective in enhancing electrical conductivity. Thus, in order that the utilization efficiency may be increased, it is necessary to augment not only oxygen overpotential but also electrical conductivity. It is for this reason that in a working example of JP Kokai H8-329937 a cobalt powder and a cobalt hydroxide powder as well as a calcium sulfide (CaS) powder are added to a nickel hydroxide powder. However, if those three components are simultaneously added to nickel hydroxide, cobalt hydroxide cannot be precipitated in the area where CaS has already been adsorbed (the precipitated cobalt hydroxide is oxidized in the charging process to .beta.-CoOOH which renders the surface of particulate nickel hydroxide electrically conductive), with the result that a uniformly conductive matrix cannot be formed on the powder surface.